The Invention of Sex

This is an article I’ve written and just submitted for a science writing prize in the UK, the Wellcome Trust Science Writing Prize. If you’re interested in entering, unfortunately the deadline was today, but there’s always next year, details are found through the link just above. The article is about why, and roughly when, sex was invented. At least on Earth…

Every generation thinks they invented sex, and it’s an established physical law that it’s simply impossible to conceive of your parent having sex. Of course, deep down we know this isn’t the case, we know sex was happening long before us, and that at some point our parents must have indulged, at least once. But back in the depths of geological time one generation did invent sex.

Sex is actually pretty important. We’ve devoted countless plays, songs, reality-TV-shows and even wars to it, and without it, we’d quickly cease to exist, its that important. But for all of the attention we lavish upon sex, few of us know when it was invented and why it became so popular?

Before sex, reproduction was a much more prosaic affair. Organisms grew till they attained a certain mass then simply split into two or more parts, each of which was a fully independent clone of the original. Such ‘asexual reproduction’ is still carried out today by most organisms, mostly microscopic life-forms like amoeba and bacteria, but also by a number of animals and many plants, ironically including the aphrodisiacal strawberry, which can create offshoots, called stolons, which grow into independent copies of the original plant.

Asexually explicit; a strawberry plant reproducing asexually

At a point in the history of life at least one pioneering species struck on a novel innovation though. Rather than cloning themselves they began to swap genetic material with other members of their species, which could then be gestated and developed into a new individual. This innovation, branded sexual reproduction, became rather fashionable, and spread like wildfire through the evolutionary tree of life.

The question as to why sex became so popular has entertained biologists for some time. After all, asexual reproduction has its advantages, population size can quickly increase for asexual organisms, as each is effectively a female who doesn’t require bothersome males to bear young, and all offspring are ‘born’ as adults, without the need to undergo a process of development. Thus asexual populations can explode at rates way beyond even the most energetic species of rabbit.

Sex also carries a number of disadvantages. It’s a great way to spread disease, it usually involves a search for a mate, seduction rituals and lying about your job, and can end badly for those involved, particularly for the males of many species of spider.

Seduction rituals, exhaustive

However, sexual reproduction offers one clear evolutionary advantage. It allows greater degrees of variation to be achieved between individuals of the same species. And variation is the fuel of evolution; it provides the raw material for natural selection to do its business.

One mechanism that causes variation between individuals is mutation. As DNA sequences are copied mistakes can be made, at times leading to the creation of new processes and structures within an organism. This occurs in all species, irrespective of how they choose to reproduce, and appears to have driven much of the evolution of life for billions of years.

But sex offered new possibilities for variation, as the genes from two parents are split and recombined in their offspring, leading to the creation of novel genomes and a greater degree of difference between individuals of the same species. This presumably endowed such species with a greater stock of pre-adaptive evolutionary innovations to allow them to better conquer environmental adversities, and gave rise to the process of ‘true’ speciation witnessed today in plants, animals and fungi.

So when was sex actually invented? And what sort of fossil evidence would be required to prove sex was occurring?

Mite sex, from 40 million years ago, at least I think they’re having sex

The British palaeontologist Nicholas Butterfield believes that the best evidence for sex is to be found in multicellularity. For over 2 billion years all life was single-celled, but likely around 1.7 billion years ago life became multicellular. This allowed cells to become specialised, to adapt unique shapes and functions, allowing more structurally complex life to emerge. Butterfield makes the case that sexual reproduction was the vital step required to kick-start multicellular cell differentiation, as the processes required for sexual reproduction are similar to those required for the development of specialised cells. Thus where you find differentiated cells you’ll find sexual reproduction

In 2000 Butterfield described a 1.2 billion year old algae-like multicellular fossil with clear cell specialisation. Furthermore, it had what appeared to be spore-like structures that allowed different male and female specimens to be indentified. This is arguably the earliest multicellular fossil discovered with specialised cells, and is considered to be the earliest evidence of a sexually reproducing species.

Bangiomorpha pubescens, in all its fossilized glory

In true smutty British style, Butterfield named the fossil specimen Bangiomorpha pubescens, pubescens from puberty, as life on Earth was passing through its evolutionary infancy and emerging into its sexual maturity, and Bangiomorpha, partly because it resembles a modern red algae called Bangia, and partly, well, I’ll leave that to your imagination.

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